You don't say what R2 is, but assuming it's much larger than Rds(on), the problem is that the MOSFET is not fully "on", and that's usually because the gate voltage is not enough. An N-channel MOSFET might be a better choice, btw, since it will have a lower Rds(on) all else equal.

A diagram with labeled components would be nice, so we don't have to guess.